The midbrain lies deep within the brain and has an important role in reward motivation, movement, and pathophysiology. Dopaminergic neurons (i.e., neurons whose primary neurotransmitter is dopamine) are comparatively few in number in the human brain and their cell bodies are confined to a few relatively small brain areas such as the substantia nigra and ventral tegmental areas of the midbrain. Dopaminergic neurons in the substantia nigra and ventral tegmental areas project to numerous areas in the brain and exert powerful effects on their targets. Impairments in the dopaminergic function of these areas of the midbrain have been implicated in various neuropsychiatric disorders such as Parkinson's disease, schizophrenia, depression, and addiction. These conditions may be addressed by stimulation of the midbrain resulting in dopamine increases to stimulate neurons.
Given that substantia nigra and ventral tegmental area neurons lie deep within the brain, the primary means of influencing the midbrain has been either systematic pharmacological interventions or implantation of deep brain stimulators. Systematic pharmacological intervention is the first line of therapy for many neurological and neuropsychiatric disorders. Currently, treatment for such conditions involves pharmacological intervention to increase dopamine levels. Unfortunately, a pharmaceutical's effect is difficult to localize and therefore increases dopamine to all areas of the brain including non-affected sections, precluding region-specific interventions. Further, some patients with these disorders do not respond to the drug therapy.
For subjects that do not respond to pharmacological intervention, invasive deep brain stimulation may be considered. However, since the midbrain is deep within the brain, such stimulation techniques require invasive procedures to insert the stimulator, thus disrupting normal functions and increasing risk to the subject.
The two predominant means of non-invasively stimulating the brain are transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). Transcranial magnetic stimulation involves inducing an electrical current within the brain via a pulsating magnetic field applied by an induction coil stimulator placed above the scalp. Transcranial direct current stimulation involves the application of a small current between anodal and cathodal electrodes placed on the scalp. Both of these stimulation methods result in changes in brain function by causing neurons' resting membrane potential to depolarize or hyperpolarize.
There is therefore a need for a method of brain repair that non-invasively stimulates neurons in midbrain regions. There is a further need for a system that allows stimulation of the prefrontal cortex in order to utilize indirect and direct connections to activate and deactivate the midbrain area.